Problem: The scale on a map is 8cm : 3km. If the distance between two cities is 30km, how far apart in cm are the two cities on the map?
The scale means that every 8cm on the map corresponds to an actual distance of 3km. An actual distance of 30km is the same as 10 $\cdot$ 3km. The distance between the two cities on the map is 10 $\cdot$ 8cm, or 80cm.